Spray painting employing high voltage charging



April 2, 1968 D. c. WHITAKER ET AL 3,376,156

SPRAY PAINTING EM?LOYING HIGH VOLTAGE CHARGING 3 Sheets-Sheet 1INVENTORS. flag/as CMiffi/7?r& T/immrl dail /1, wa

N QNNN Filed March 19, 1964 A TTORIVEYJ.

April 2, 1968 D. c. WHITAKER ET AL 3,376,156

SPRAY PAINTING EMPLOYING HIGH VOLTAGE CHARGING 3 Sheets-Sheet :3

Filed March 19, 1964 IN VENTORS. flaayis' W fiiid/Tenfi 7/10/2145 J647110,

A 7708 E Y6 N uQNaH April 1968 D. c. WHITAKER E L 3,376,156

SPRAY PAINTING EM?LOYING HIGH VOLTAGE CHARGING Filed March 19, 1964 3Sheets-Sheet 3 M M n j 5 12! 53 Flfi 3 9 5 5.9

l l "w i l a A TTOENE Y5- United States Patent 3,376,156 SPRAY PAKNTENGEMPLQYHNG HIGH VQLTAGE CHARGING Douglas C. Whitaker, Greenwood Ave.,Wyncote, Pa. 19095, and Thomas J. Gallen, 1516 Buck Road, Feasterville,Pa. 19047 Filed Mar. 19, 1964, Ser. No. 353,051 6 Claims. (Cl. 117-932)ABSTRACT OF THE DESCLOSURE A method and apparatus for spray paintingwork pieces is disclosed in which atomized paint particles are ejectedfrom a grounded spray gun and directed at, or into the path of,inductively charged work pieces.

This invention relates to spray painting.

A particular object of the invention is to provide an improved methodand apparatus for spraying paint (or other coating materials) on toshaped (or other) articles to form uniformly even coatings of suitablethickness, and wit-h a minimum waste of paint.

In accordance with our present invention, the spray painting isperformed in a charged high-voltage atmosphere. However, as will bepointed out later, there are a number of important differences betweenthe spray painting system of the present invention and the electrostaticspray painting systems of the prior art.

Referring now to the drawing;

FIG. 1 is a diagrammatic side elevational view of a preferred form ofspray painting apparatus according to the present invention;

FIG. 2 is a diagrammatic and elevational view looking in from the rightin FIG. 1; and

FIG. 3 is an enlarged elevational view looking along the line III-I IIof FIG. 1.

Referring now to the drawing, the reference numeral represents astructural housing or booth which is divided by a through passage into afront portion 10F and a rear portion 10R. Mounted in the front portion10F are one or more spray guns 12 and associated equipment. Three guns12, 12a and 12b are shown in FIG. 1. Each gun is mounted, as on bench40, to point rearwardly toward an exhaust grill 13 in the rear portion10R through which air is drawn from the passage 15 and exhausted in acontrolled manner, as by an exhaust fan not shown, into the exhaust duct14.

An overhead conveyor passes through the passage 15 from which aresuspended, at spaced intervals, the articles or pieces of work to becoated. The overhead conveyor, which may be of any suitable type, isillustrated as comprising an overhead channel or track 16 in the form ofa cylindrical pipe or tubing having therein an endless chain 17 which ispulled through the track 16 by any suitable drive means, not shown. Theunder surface of the track 16 is provided with a continuous slot throughwhich protrude a plurality of eyes 18 which are secured to the chain 17at spaced intervals.

Secured to and supported by each of the chain eyes 18 are a group ofsupporting and insulating components connected in series and from thelower end of which the work piece 27 is suspended. In the illustratedembodiment, the series of components includes, starting at the top thehook rod 19 having thereon the roller 20, the U-hook 21, thedouble-ended hook 22, the insulator support arm 23, the insulator 24,the angle member 25, and the work hook 26 from which the work piece 27is suspended. The rod 19 has a hook at its upper end for inserting intothe hole of the chain eye 18. Rotatably secured to the lower end of rod19 are a pair of cylindrical metal cups in back-to- Patented Apr. 2,1968 back relation, forming the roller 20 adapted for rotation about theaxis of the rod 19. Secured to the lower cup of the roller 20 is aU-hook 21 which turns rotationally when roller 20 is rotated. Suspendedfrom the U-hook 21 is a double-ended hook 22 from which is verticallysuspended a dependent metal support arm 23. To the lower end of themetal support arm 23 is secured a rodlike insulator 24, to the lower endof which is secured a metal angle member 25, having a hole in itsvertical portion for receiving the double-ended metal hook 26 from whichis suspended the work 27 to be painted. In the drawing, the work to bepainted is illustrated as being a rectangular metal frame 27, but thisis merely illustrative since the work could have many different shapes.

A beam 28 is fastened to the ceiling of the booth 10 at a location closeto the conveyor track 16 and in a position which parallels the track 16at a small spaced separation therefrom. Secured to the under surface ofbeam 28 is an angle member 29, having a vertical depending portion, thesurface 30 of which, facing the roller 20, is provided with an abrasivecoating or is otherwise roughened, so that as the conveyor is moved theroller 20 frictionally engages the abrasive or roughened surface 39 andis caused to rotate, thereby causing the rotation of all of the partswhich are suspended from the U-hook 21, namely, the double-hook 22, themetal support arm 23, the insulator 24, the angle member 25, the supporthook 26, and the work piece 27, all of which rotate continuously as theconveyor chain moves through the booth 10.

A high-voltage wire 31, supported at each side wall of booth 10, is somounted as to extend through passage 15 below the level of the insulator24, preferably just below the lower end thereof. The location of wire 31is very close to the path traveled by the members 25. The preferredspacing between wire 31 and the members 25, as they are moved throughthe booth by the conveyor system, is of the order of 1"1 /2".High-voltage wire 31 is supported at each side of the booth by thehigh-voltage insulators 32 and 33. One end of Wire 31 (the right end inFIG. 2) is connected to a high-voltage supply, which may take anysuitable form, and may be of the order of 40,000-100,000 volts DC. Inthe drawing, the high-voltage supply is illustrated as being comprisedof power line cable 34, the support insulator 35, the feed wires 36, andthe high-voltage transformer and rectifier 37.

The other end of the high-voltage wire 31 is merely terminated at theinsulator 32; this end of wire 31 is not grounded. Thus, wire 31 is at ahigh potential relative to ground.

The spray guns 12, 12a and 1212 may be of any known form of atomizer towhich liquid paint is fed and which spray out atomized paint. Each gunmay preferably be a centrifugal atomizer to which liquid paint andcompressed air are sup-plied and which projects a hollow cone-shapedcloud 39 of atomized paint as depicted in FIGS. 2 and 3. In FIGS. 2 and3, liquid paint may be assumed to be fed to gun 12 through the piping 42and compressed air through the hose 43.

In accordance with our present invention, the movement of air from thepassage 15 of the booth 10 through the exhaust grill 13 and out theexhaust duct 14 is carefully controlled, so that the fog or cloud ofatomized paint particles, produced by each gun, tends to move slowly inthe direction of the exhaust grill 13. The guns 12, 12a and 12b aremounted at such a level that the cloud of atomized paint particlesproduced by each gun lies in the path traveled by the work 27 as it isconveyed through the passage 15.

It is important to the operation of the apparatus of the presentinvention that the spray guns be grounded, as is,

3 indicated at 41 in FIG. 3. This may be done by a direct lead from thegun to ground, or by conductively mounting the gun or guns on a metalbench 40 of good conductivity, and grounding the table. The conveyortrack 16 is also grounded, as indicated at 44.

Since the terminal end of the high-voltage wire 31 is insulated fromground by insulator 32, the wire 31 is at a high potential substantiallyequal to the voltage applied to its'input end 33. This applied voltagemay preferably be of the order of 40,000-100,000 volts DC. A strongelectrostatic field thus surrounds the wire 31, and as the work pieces27 carried by the conveyor move through the booth 10, each work pieceacquires by induction a high potential relative to ground. The workpiece thus attracts the atomized particles of paint in the suspendedcloud 39, and particles of paint move toward the work piece from alldirections, as is indicated in FIG. 3 by the arrows. In this manner, thearticle 27 is coated with a coat of paint, evenly and uniformly.

By establishing a proper relationship between the rate of movement ofthe articles 27 through the booth and the amount of atomized paintprojected from the gun or guns and formed into the suspended cloud orclouds 39, having due regard for the size and surface area of the work,a large proportion of the particles of paint projected from the guns maybe attracted to and coated on to the work, with little paint being lostby exhausting on to or through the exhaust grill 13.

The important features of the apparatus of the present invention, asdescribed above, and its manner of operation, include the following:

(1) The spray gun or guns are at ground potential. The guns are notcharged to a high voltage, and thus the particles of paint are notcharged by the gun, as is the case in some prior art electrostatic spraypainting systems.

(2) The paint ejected from the gun or guns is atomized by compressed airand/ or by centrifugal forces. The paint is not atomizedelectrostatically by a high-voltage spray gun.

(3) The movement of air in the vicinity of the projected particles ofatomized paint is carefully controlled to cause a cloud or fog of paintparticles to move slowly across the path of the articles to be painted.

v (4) The articles of work to be painted are passed through the slowlymoving cloud of paint particles.

(5) A high-voltage conductor is placed in the vicinity of, but out ofcontact with, the work. This high-voltage conductor is also locatedoutside of the cloud of atomized paint particles.

As a result of the features listed above, a number of advantages arederived, which include the following:

(1) As the work is passed through the slowly moving cloud, particles ofpaint are drawn to the surface of the work from all directions, therebyelfecting wrap-around and thereby providing a uniform and even coatingof paint on the article even though it be of irregular shape.

(2) As contrasted with prior art systems, there is considerably lessoverspray and loss of paint. The atomized particles of paint, being atground potential as they leave the grounded gun, tend to pick up acharge as they move toward the work piece and toward the electrostaticfield. In prior art systems in which the particles of paint are chargedby reason of the spray gun itself being charged, the work is grounded,and reliance is placed upon the potential difference between the chargedpaint particles and the grounded work to deposit the particles of painton the work. It seems, however, that a charged particle of paintapproaching a grounded piece of work tends to lose its charge much morerapidly than a particle of paint which leaves the gun uncharged andapproaches a charged piece of work.

(3) The fact that the spray gun itself is unchanged enables the systemof the present invention to handle paint materials which are highlyconductive, as for example, paints which contain powdered metal. In theprior systems in which the spray gun itself is at high potential, if ametalized or other conductive coating of paint is used, the charge onthe gun is carried back through the conductive fiuid in the paint supplyline to the container or other source of conductive paint supply, andthe supply container assumes a dangerously high potential. This makes itnecessary to insulate the paint container and the pumps from ground.Hence, whenever it is necessary to replenish the supply of paint in thecontainer, it is necessary to cut off the high voltage. Moreover, it issometimes desirable to heat the paint before supplying it to the gun. Inthe prior art systems in which a charged gun (5) In a preferred form,where centrifugal atomization is used, a Faraday screen in the form offorwardly projecting fine rigid wires 12c secured to and rotatable withthe gun disc insures against electrostatic atomization at the leadingedge of the spray head.

(6) Grounding the spray gun and its supply lines permits the use ofcompressed air in combination with mechanical energy as a means foreffecting atomization without the need for isolating the electricalsystem. The pieces of work to be painted become charged by thehighvoltage conductor as soon as they enter the spray booth, which isprior to entering the actual spray zone, and this high potential chargeon the work is maintained so long as the work is within the booth. Thisassures a definite potential difference between the pieces of work tobepainted and the atomizing spray gun or guns. The pieces of work becomecharged to a high potential by induction, due to the existence of apotential gradient between the high-voltage wire and the work, andbetween the work and the grounded spray gun. This arrangement has beenfound to eliminate virtually all ripple, assuming that a rectified andfiltered D.C. potential is applied to the high-voltage wire.

(7) In the apparatus of the present invention, there are no grids inclose proximity to the spray cloud to collect the overspray.

(8) The fact that the particles of atomized paint are not charged at theinstant they leave the gun enables the air exhaust system to exert abetter control on the advance of the spray cloud in the direction of thework path, since there is no initial forward drive of the paint at theinstant that it leaves the gun.

While the work pieces 27 illustrated in the drawing have been describedas rectangular metal frames, it is not essential that the work be metalor conductive. The apparatus of the present invention is perfectlycapable of painting or coating non-conductive articles, such as articlesof glass or porcelain. In such cases, however, the paint used should beconductive, preferably either a metallic paint or a water-base paint.

It has been stated hereinabove that the spray gun may be any knownsuitable type. One suitable form is a gun having a centrifugal sprayhead wherein the liquid paint is positively discharged against aplurality of annularly arranged accelerator members which are rapidlyrotated by an air driven motor. The paint is thereby atomized creating aspray which is projected forwardly of the head and shaped into a patternby the dished portion of the head which extends beyond the point atwhichthe paint is mechanically atomized. The complete head is groundedelectrically.

Having described our invention, we claim:

1. The method of spray coating 0. work piece comprising the steps of;

(a) creating an electrostatic field between an elonggated elevatedhorizontally-disposed high-voltage wire and ground,

(b) inductively charging the work piece to a potential substantiallythat of said high-voltage Wire by supporting the work piece conductivelyat a remote distance from the high-voltage wire and moving theconductive support means along a path close to and parallel with theelongated high-voltage wire while connecting the conductive supportmeans to ground through an insulator member,

(c) and projecting a cloud of atomized particles of coating materialfrom a grounded gun into the path of the inductively charged work pieceto effect deposition of said atomized particles of coating material onthe work piece.

2. Spray coating apparatus comprising:

(a) an elongated high voltage conductor supported by insulator means ina generally horizontal position at a selected elevation above ground,

(b) high voltage supply means connected to said conductor,

(c) a grounded overhead conveyor having dependent hanger means forsupporting the work piece to be coated,

(d) said hanger means including a conductor portion connecting to saidwork piece and an insulator portion between said conductor portion andsaid conveyor,

(c) said conveyor conveying said conductor hanger portion along the pathwhich parallels and is close to said high voltage conductor but out ofcontact therewith for charging the work piece inductively;

(f) and grounded spray means for projecting a cloud of atomizedparticles of coating material into a zone which does not include thehigh-voltage conductor but which is in the path of the inductivelycharged work piece.

3. Apparatus according to claim 12 characterized in that said conveyoris provided with a plurality of said hanger means at spaced intervals.

4. Apparatus as claimed in claim 3 further characterized in thatair-movement control means is provided comprising exhaust means disposedon the opposite side of said work path from said spray means, forcontrolling the movement of said cloud across said work path.

5. Apparatus as claimed in claim 4 further characterized in that saidconveyor includes means for rotating said work piece as it passesthrough said zone.

6. Spray painting apparatus comprising, atomizing means for projecting ashaped spray of atomized paint into a zone; exhaust means controllingthe air movement in said zone; a conveyor having a plurality ofspacedapart elongated depending carriers for suspending work piecestherefrom and for transporting said work pieces through said zone, eachof said carriers including conductor sections and an insulator section,the locus of said moving carriers being a vertical plane transverselydisposed forward of said atomizing means and close thereto; a singlehigh voltage wire disposed generally horizontally and parallel to theplane of movement of said carriers and a short distance therefrom, saidwire extending to both sides of said zone below said insulator sectionand above the work piece; means for applying a high voltage to one endof said wire; and means terminating said other end of said wire inspace.

References Cited UNITED STATES PATENTS 2,421,787 6/1947 Helmuth 11793.432,425,652 8/1947 Starkey 117-93.44 2,428,991 10/1947 Ransburg 11793.42,442,986 6/ 1948 Ransburg 1 17-93.44 2,446,953 8/1948 Ransburg 117-93.4X 2,463,422 1/ 1949 Ransburg et al. 11793.43 2,730,460 1/1956 Ransburg11799.44 2,730,988 1/1961 Starkey 11793.44 X 2,967,331 1/1961 Kaspar1179.3.4 3,113,037 12/1963 Watanabe 117-93.4 2,773,472 12/1956 Lamm239-15 A. GOLIAN, Assistant Examiner.

